Causal systems often include mutually exclusive events: events which cannot occur simultaneously. However, when events in a causal system are exclusive, the normative properties of the whole system change substantially. Are adults sensitive to the consequences of exclusivity for causal reasoning? Here, we systematically manipulated common-effect causal systems to have either exclusive or non-exclusive causes while holding all other factors constant. Adults showed a rich understanding of exclusive systems in making both predictive (Experiment 1) and diagnostic (Experiments 2 and 3) causal inferences. Adults’ success in these tasks suggests that exclusivity is an important dimension in human causal reasoning.